1. Field of the Invention
This invention relates to a method and apparatus for monitoring the position of a workpiece tube in a pilger forming operation over time, and in particular to a device which monitors axial, rotational and transverse displacement of the tube, to enable detection of set-up problems, lubrication deficiencies and process variations indicating a predisposition to wear of the pilger forming apparatus and/or the results of wear which already has occurred.
2. Prior Art
A pilger mill is a forming apparatus for axially elongating and reducing the diameter of a ductile tube. A pair of opposed rollers or dies bear on the tube over a forming area covering an axial length, and have tapering grooves around a portion of their circumference. The dies are rolled over the tube or workpiece in the forming area to incrementally reduce the tube diameter while the tube is supported on a mandrel. The dies reduce the external diameter, and in conjunction with the mandrel elongate the tube axially while thinning the tube walls. The dies are operated in a cyclic manner. With each stroke the tube is advanced along its longitudinal axis and rotated.
Pilger forming is used in the production of precision tubing such as zirconium alloy tube as used in cladding nuclear fuel. This process involves rolling the tube over a tapered mandrel between two semi-circular grooves machined in the periphery of two opposed dies to form a very thin walled tube to precise dimensions. The input tube or workpiece is incrementally fed along its axis and rotated with the tapered mandrel between each work stroke and return stroke of the dies. The machine operates at up to about 250 work and return stoke cycles per minute, and produces in excess of 80% area reduction in the tubular work piece. Some examples of pilger forming for zirconium alloy tubes are disclosed in U.S. Pat. Nos. 4,233,834 - Matinlassi; 4,674,312 - Cook; 4,765,174 - Cook et al; 4,819,471 - Cook; and 4,990,305 - Foster et al.
Because of the precise dimensional tolerances required particularly in the zirconium alloy product and the high cost of the equipment and tooling, it is important to identify as early as possible any conditions that could lead either to deterioration of the equipment or to the production of unacceptable product. Some conditions which produce unacceptable product are poor lubrication, tooling misalignment and excessive equipment wear. It may be possible to detect some of these conditions by quality control measurement steps conducted on finished product. However, conditions which cause wear or produce unacceptable product cause irretrievable damage to workpieces in progress and/or to the forming apparatus.
It is known to instrument rolling mills in connection with controlling the motion of the rolls or dies. Such apparatus are disclosed, for example, in U.S. Pat. Nos. 3,948,070 - Hentzschel et al; and 4,038,848 - Ichiryu et al. Instrumentation can also be used to identify misalignment of the rolls and improper roll gap, using sensors which engage with the rolls or the drive shafts on which the rolls are mounted. Examples are disclosed in U.S. Pat. Nos. 4,044,580 - Worden et al; and 4,059,794 - Furness et al. However, whereas some defects in operational status of the rolls may occur when the rolls are properly aligned a better and more direct way to identify problems would be advantageous.
In various production processes, a variety of measurements are taken during the progress of particular pieces through the production steps. Upon completion of production, process engineers and the like attempt to correlate attributes of the finished product with measurements taken during production. This enables identification of the particular process parameters which affect the attributes of the finished product. For example, via quality assurance measurements it may be discovered that a particular run of formed product is characterized by undue bowing, eccentricity of the lumen of the formed tube or the like. These attributes may be correlated with the use of particular forming equipment such as dies which are worn or improperly set up. However, correlating the finished product with previous production steps obviously fails to correct problems until a quantity of defective product already has been produced. An earlier and more direct means to identify problems in finished workpieces is needed, preferably a means which can identify a problem even as to a workpiece in the progress of being formed.
The present invention provides a system whereby the attributes of the workpiece are measured to identify problems as they occur. The invention thus provides a means for directly sensing product attributes in a manner that reflects either or both of problems in the forming apparatus and the product which is formed.
The invention in particular provides a means for acquiring and displaying data respecting the transverse displacement of a workpiece in successive forming cycles of the dies in a pilger forming process. By displaying the displacement with respect to mutually perpendicular axes, it is possible to visualize the extent to which the workpiece is experiencing unequal lateral forces, which are characteristic of problems which result in undue wear or unacceptable finished product dimensions. This capability is preferably provided on-line and substantially in real time during the forming process.